1. Technical Field of the Invention
This invention relates to overhead projectors and, more specifically, to an improved system of overhead projection which forms an undistorted image on a vertical surface or screen.
2. Brief Description of the Prior Art
Conventional overhead projectors include a stage which supports the transparency to be projected, lighting means mounted below the stage to illuminate the transparency, and a projection lens and mirror combination suspended above the stage to direct the light image of the transparency onto a viewing screen. If the stage and projection lens are in parallel planes; the centers of the projection lens, the stage and the light source are on a common axis; and the mirror is oriented at a 45.degree. angle to the plane of the stage, an undistorted image will be projected on a vertical screen. However, if these conditions are met, at least half of the projected image will be below the level of the mirror and, consequently, the audience's view of the image will be obstructed by the projector.
To provide the audience with an unobstructed view, the mirror is commonly tilted to an angle greater than 45.degree. with respect to the horizontal, thereby raising the projected image above the level of the projector. If the viewing screen remains vertical, this projection of the image at an oblique angle above the horizontal results in a magnification distortion of the image into a trapezoidal shape in which the upper portion of the image is wider than the lower portion. Because of the shape of the projected image, this magnification distortion is commonly referred to as "keystoning".
A second type of distortion common to such projectors is a lack of uniform focus throughout the projected image. This type of distortion will occur unless the so-called Scheimpflug condition is met which requires that the plane of the screen and the planes of the virtual images of the projection lens and the stage be parallel or converge to a common point.
It is claimed that oblique projection of the image may be achieved without introducing either keystone distortion or focus non-uniformity distortion by tilting some of the elements which comprise the projector. U.S. Pat. No. 4,002,408 which issued to Amma on Jan. 11, 1977, for example, discloses two methods by which it is claimed that distortion-free oblique projection may be achieved. The first method involves moving the projection lens and mirror away from the center of the stage toward the screen and tilting the lighting means to project light at an angle through the transparency to the projection lens and mirror. The second method requires that the projection lens and the stage be tilted with respect to the mirror and the illuminating apparatus. Both methods introduce mechanical complexities to the projector and a tilted stage would pose difficulties for the presenter in positioning the transparency on the stage and writing on the transparency as it is being projected.